1. Field of the Invention
The present invention relates to a hydraulic control device for an automatic transmission, in particular to an oil passage structure for a frictional engaging element having an accumulator.
2. Description of the Prior Art
Conventionally, an oil passage `a` which is connected to a frictional engaging element such as a first (forward) clutch hydraulic servo C1, as shown in FIG. 8 is communicated with an accumulator 10 branched from the passage a, and upstream of the branched passage c, a check ball 5 and an orifice 6 are placed.
Due to the above arrangement, when hydraulic pressure is supplied to the oil passage a, the pressure is supplied to the hydraulic servo C1 and the accumulator 10 through the orifice 6. In accordance with the characteristics of the accumulator 10, the hydraulic pressure for the servo C1 rises, so that the hydraulic servo C1 is engaged smoothly.
When the oil passage a is drained, the hydraulic pressure in the servo C1 and the accumulator 10 are drained through the check ball 5.
With reference to the above mentioned structure, when the pressure is drained from the hydraulic servo C1, the draining pressure from the servo C1 is influenced by the draining pressure from the accumulator 10, which has comparatively much volume. Because of the above, though the draining is conducted through the check ball quickly, the hydraulic pressure of the servo C1, as shown in dotted lines in FIG. 9, has a draining characteristic H1' because the resistance of the oil passage caused by much oil flow from the accumulator 10.
In particular, in case that the hydraulic servo is a servo for a first (forward) clutch C1, when a manual shift lever is operated from the forward D range to the reverse range R through the neutral range N suddenly before the servo C1 is completely drained, the line pressure from the reverse port is supplied to a frictional engaging element which is engaged at reverse running (H.sub.R). Under such condition, the first clutch hydraulic servo and the frictional engaging element for reverse running are both under torque-transmitting state. So, as shown in FIG. 9 by the dotted lines, transmission torque Q' is suddenly changed, so that shift shock may occur.